1. Field of the Invention
The present invention relates to an aqueous cementing composition and method of using same in cementing oil and gas wells and the like. More particularly, the present invention concerns incorporation of a polymer prepared by the polymerization of monomers or salts of monomers of 2-acrylamido-2-methylpropanesulfonic acid, styrene and acrylic acid in a hydraulic cement which may contain substantial salt concentrations for the purpose of reducing fluid loss during cementing operations.
2. Description of the Prior Art
Cement compositions are used in the oil and gas industry to cement the annular space in the wellbore between the surrounding formation and the pipe or casing. Typically the cement slurry is pumped down the inside of the casing and back up the outside of the casing through the annular space. The amount of water which is used in forming the cement slurry will vary depending upon the type of hydraulic cement selected and the job conditions at hand. The amount of water used can vary over a wide range, depending upon such factors as the required consistency of the slurry and upon the strength requirement for a particular job. Many times, the hydraulic cement must be placed within or next to a porous medium, for example earthen strata in the wellbore. When this happens, water tends to filter out of the slurry and into the strata during placement and the setting of the cement. Many difficulties are related to an uncontrolled fluid loss of this type such as an uncontrolled setting rate, improper placement of the slurry, impaired strength properties, and a contamination of the surrounding strata. These conditions are all undesirable in oil and gas well cementing operations.
Certain polymer compositions have long been recognized by those skilled in the art of cementing wells in the petroleum industry as cementing additives useful in reducing fluid loss from a slurry of cement and water to the surrounding environment, such as, the formation. These compositions are commonly referred to as "fluid loss additives".
Discussions of the use of fluid-loss additives in well cementing compositions and their importance may be found in the following articles, the disclosures of which are incorporated herein by reference:
A. Carter, Greg and Slagle, Knox, "A Study of Completion Practices to Minimize Gas Communication", Society of Petroleum Engineers Paper No. 3164, November 1970; PA1 B. Christian, W. W., Chatterji, Jiten, and Ostroot, Warren, "Gas Leakage in Primary Cementing--A Field Study and Laboratory Investigation", Society of Petroleum Engineers Paper No. 5517, October, 1975; PA1 C. Cook, C. and Cunningham, W., "Filtrate Control: A Key in Successful Cementing Practices", Journal of Petroleum Technology, August 1977, page 951; PA1 D. Smith, Dwight, Cementing: SPE Monograph Volume 4, published by Millet the Printer, Inc., Dallas, Tex., 1976.
An example of a fluid loss additive for use in an acidizing or fracturing composition is found in U.S. Pat. No. 4,107,057. In the '057 patent a copolymer of a sulfonic-acid modified acrylamide and a polyvinyl cross-linking agent is employed.
In the oil well cementing art, a variety of polymers have been disclosed as useful fluid loss additives for hydraulic oil well cements. For example, U.S. Pat. No. 4,015,991 discloses such a fluid loss additive for a hydraulic cement slurry consisting of hydrolyzed copolymers of acrylamide (AM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS.RTM., a trademark of The Lubrizol Corporation). However, these AM/AMPS.RTM. copolymers are useful only in operations where the bottom hole circulating temperature (BHCT) ranges from 90.degree. to 125.degree. F., whereas BHCT ranges encountered in such operations are often outside such a range. Still further, these copolymers have a salt tolerance of only up to about 10% by weight of water.
The temperature limitations of the AM/AMPS.RTM. copolymers, that is, loss of usefulness above about 125.degree. F. BHCT, are believed to be the result of hydrolysis of the amide groups. The carboxylate groups formed by such hydrolysis convert the copolymers to materials which function to retard the setting of the cement and to reduce the compressive strength of the set cement. Further, in the lower portion of the above-mentioned temperature range (between 90.degree. and 100.degree. F.) the AM/AMPS.RTM. is less effective as a fluid loss additive, requiring inclusion of larger amounts of such additive than at higher temperatures. The inclusion of a sufficiently large amount of additive to create an acceptable fluid loss composition often creates viscosity and pumpability problems, since the addition of such copolymer directly affects the resultant slurry rheology. Copolymers of acrylamide and AMPS.RTM. exhibit high viscosity and poor mixability, resulting in cement slurries having poor pumpability characteristics during cementing operations. Mixability is a subjective term used to describe how well the components in the cement composition wet and mix with each other, as well as the energy required to create a generally homogeneous slurry.
Industry desires a fluid loss additive that has as little effect on compressive strength, set time, viscosity and thickening time as possible; is salt tolerable, that is, does not exhibit substantial loss of effectiveness in the presence of salt; and is chemically stable during cementing operations. Further, such desired fluid loss additive should be compatible with as many other additives and environmental conditions as possible, should be soluble in cement slurries at normal ambient temperatures encountered in oil well cementing operations, as well as continue to provide fluid-loss characteristics over a broad temperature range and at the high pH of the cementing compositions.
U.S. Pat. No. 4,515,635 discloses the use of copolymers of N,N-dimethylacrylamide (NNDMA) and AMPS.RTM. having monomer ratios of NNDMA to AMPS.RTM. of 1:4 to 4:1 and selected molecular weights as fluid loss additives for cement compositions. As illustrated in the patent, however, the copolymer fails to provide desired levels of fluid loss control even at relatively high concentrations to cements containing substantial salt concentrations.
It would be desirable to provide additives for use in reducing fluid loss in cement compositions, and particularly, those containing substantial salt concentrations where the bottom hole circulating temperatures may range from about 100.degree. F. to in excess of about 400.degree. F. without adversely affecting thickening time, compressive strength or other properties of the cement composition.